1. Field of the Invention
This invention relates to photographic cameras.
More particularly, the invention relates to a camera wherein a film transporting mechanism is operated to advance a film (or film section) by the length of picture.
2. The Prior Art
Various proposals have been made for transporting film in still cameras, including in the types of cameras which are generically known as instant-picture cameras. It has been suggested to use a spring- or electrically-operated transporting mechanism, a rotary film-transporting knob, a pivotable lever, a crank which is turned by hand when film is to be transported, or a slidable element.
In one camera which is collapsible, i.e., where a lens mount is movable relative to the camera body between a collapsed (shut) and an extended (open) position, the proposal has been made to effect film transportation as a function of the movement of the lens mount relative to the camera body. This can be done by, e.g., providing an arcuate gear segment which pivots with the lens mount about a pivot axis thereof and which drives a step-up gearing.
The problem with all proposed solutions is that they do not--or not adequately--take account of the fact that different film-transporting forces are required during different film-transporting phases. This difference in force requirements during different phases of each film-transporting cycle is particularly pronounced when the transporting function is coupled with one or more other functions, i.e. when it is desired that the same force which effects film transportation should also, e.g., cock the camera shutter, flip a mirror of the camera or tension (to store energy) a device which ignites the film flash arrangement (e.g. by impact upon a piezoelectric element).
If the camera is of the type having manual film transport (e.g. via a turnable knob or pivotable lever), it was heretofore simply left to the user to exert greater force during that phase of the film transporting cycle during which such force was needed. If the film transporting mechanism was motor driven, the motor was so constructed that it was able to furnish the maximum torque required of it. Of course, this meant that when the end of, e.g., a roll of film was reached, the motor was then also powerful enough to tear the film end loose from the film cassette or film cartridge and/or to tear the film sprocket holes.
Instant-picture cameras have particular problems with the matter of different film-transporting forces during different phases of the film-transporting cycle. In these cameras the exposed film sheet is withdrawn from a film pack and must travel through the nip between a pair of transporting rollers. These also act as developer-spreader rollers. Thus, at the beginning of the film transport (as the film sheet moves into the nip) the rollers encounter a developer-containing pouch of the film sheet and must squeeze the developer out of this pouch (requiring increased force). The subsequent transportation of the film and spreading of the developer over the exposed film area requires less force. However, toward the end of the transportation cycle the residual developer must be squeezed into a developer trap; since this constitutes a thickening of the film sheet, as does the pouch, increased force is again required. Moreover, depending upon the camera it may also be necessary to cock the shutter and/or store energy for the percussion-type flash lamp triggering mechanism.
The prior-art cameras of this type either had manual film transport--and took no note of the problems, or they used electrical devices and switched additional batteries into the drive circuit when increased torque was required (or else a mechanical speed-compensator was employed).
None of these expedients are fully satisfactory and/or reliable. They are too bulky for the compact cameras which are being more and more demanded, or they are too complicated and/or expensive.